Use the Isochron
Map (Figure 2.12 in the textbook or the attached jpeg
file) to answer the following questions. Recall that seafloor
isochrons reveal the positions of divergent (ridge) boundaries in earlier
Part I - Atlantic Ocean
1. When did the
North Atlantic Ocean first start to
2. Where is the
oldest oceanic crust found in the Atlantic
3. Has the rate
of opening of the North Atlantic ocean changed
over time or remained fairly constant?
4. When did the
South Atlantic Ocean first start to open?
5. Contrast the
rate and style of opening of the North and South Atlantic.
Part II - Pacific Ocean
1. The oldest
oceanic crust off the coast of South America
is about 60 m.y. old. The oldest oceanic crust off the coast of the
Phillipines is about 200 m.y. old. If the East Pacific rise is creating
this oceanic crust in a symmetric pattern, then what happened to the old
oceanic crust in the eastern Pacific?
2. The boundary
between the North American Plate and the Pacific plate is currently a transform
fault (San Andreas fault). The evidence
in the isochrons indicates that the plate boundary was different in the
past. What type of plate boundary or boundaries existed in the
past? How do you know this? When did the change in plate boundaries
III - Reconstruction
1. Make a copy
of the isochron map (Black and White is OK) and remove seafloor that is less
than 40 Ma (Red and Orange
2. Make a
reconstruction of the Earth at 40 Ma by putting the remaining map pieces back
together at the ridge boundaries assuming that Antarctica
is stationary. Remember that the Earth isnít getting smaller so there
will be gaps in your reconstruction representing subducted lithosphere.† Also keep in mind that some plate boundaries
from 40 Ma do not exist today.† So you
may need to cut your map into more pieces in order to get everything to fit
3. What does
the reconstruction tell you about the relative (to Antarctica)
motion of the continents and the plate boundaries? Can you suggest
another frame of reference for the reconstruction? Why?
Study Box 1-8
(attached).† Be sure you understand what
is happening to plate B before you begin this exercise.
***Hard copy only available***